Derivatives of 1-oxyl-4-hydroxy-2,2,6,6-tetramethylpiperidine

ABSTRACT

Selected derivatives of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine are surprisingly effective as inhibitors to prevent the premature polymerization of acrylic and methacrylic acids, their esters, their amides, vinyl acetate and acrylonitrile in the presence of water. Some of these derivatives are new compounds.

This is a divisional of application Ser. No. 09/095,617, filed Jun. 10,1998, now U.S. Pat. No. 5,932,735, which is a continuation-in-part ofapplication Ser. No. 08/876,014, filed on Jun. 13, 1997, now abandoned.

The instant invention pertains to the use of selected derivatives of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidineas inhibitors forpreventing the premature polymerization of acrylic and methacrylicacids, their esters and amides, of vinyl acetate and of acrylonitrile inthe presence of water.

BACKGROUND OF THE INVENTION

Many of the industrially important ethylenically unsaturated monomersare highly susceptible to unwanted radical polymerization initiatedeither thermally or by adventitious impurities. Some examples of thesemonomers are acrylic and methacrylic acid, acrylate and methacrylateesters, acrylamide and methacrylamide, vinyl acetate and acrylonitrile.Premature polymerization may occur during manufacture, purification orstorage of the monomer. Many of these monomers are purified bydistillation. It is in this operation where premature polymerization ismost likely to occur and to be the most troublesome. Methods to preventor reduce the amount of such polymerization are thus highly desirablesince the prevention or mitigation of such premature polymerizationincreases the yield of purified monomer and also insures against costlyand potentially dangerous runaway polymerization in the plant.

Stable nitroxides are known in the art to be effective in preventing thepremature radical polymerization of ethylenically unsaturated monomers.Some examples are seen in Japanese Hei 9-268138 which discloses thestabilization of styrene by1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine and its lower alkylethers in the presence of nitrophenols. U.S. Pat. Nos. 3,747,988describes the use of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine as apolymerization inhibitor for acrylonitrile in the presence of water andoxygen. U.S. Pat. No. 3,488,338 discloses that1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine is an effective chainstopper in the aqueous polymerization of chloroprene. British Patent No.1,127,127 describes the stabilization of neat acrylic acid by1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine. Japanese Sho 60-36501describes the stabilization of acrylate and methacrylate esters.

U.S. Pat. Nos. 5,322,960 and 5,504,243 disclose the use of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine in preventing thepolymerization of acrylic and methacrylic acids and their esters in thepresence of water, but tout the great advantages of using said oxylcompound in combination with manganese acetate, or with hydroquinone andphenothiazine.

EP 178,168 teaches the use of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine or its reaction productwith hexamethylene diisocyanate in stabilizing acrylic acid ormethacrylic acid in the presence of water.

EP 791,573 discloses that the lower alkyl or aryl esters of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine are effectivepolymerization inhibitors alone or in combination with variouscoadditives for vinyl acetate in the presence of water.

Japanese Hei 5-320205 generically describes the use of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine, its lower alkyl ethersand lower alkanoic esters in preventing the polymerization of acrylicand methacrylic acids alone, but preferably in the presence of chelatingagents for ferric salts, such as ethylenediaminetetraacetic acid. The4-hydroxy, 4-methoxy and 4-acetoxy derivatives are specificallydisclosed.

Japanese Hei 5-320217 teaches the stabilization of acrylic andmethacrylic acids with 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine,its lower alkyl ethers and lower alkanoic esters alone, but preferablyin the presence of phenothiazine, an aromatic amine or phenol. The4-hydroxy, 4-methoxy and 4-acetoxy derivatives are specificallydisclosed.

Since, during the processes to produce and purify various ethylenicallyunsaturated monomers, water is often present during one of the processsteps, there is a long felt need for the stable nitroxide inhibitor tobe sufficiently water soluble or miscible to remain homogeneous in wetmonomer streams and to prevent polymerization in the aqueous phase andyet for the inhibitor to be able to partition to such an extent that itcan prevent polymerization in both the aqeuous phase and in the organicmonomer phase for inhibition protection throughout the entire process.

OBJECT OF THE INVENTION

The object of this invention is to provide a derivative of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine of sufficient watersolubility and the concomitant ability to partition into an organicphase which will prevent the premature polymerization of ethylenicallyunsaturated monomers in the presence of water.

Another object of this invention to provide novel nitroxide compounds ofvalue in stabilizing unsaturated monomers.

DETAILED DESCRIPTION

The instant invention is to a monomer composition stabilized againstpremature polymerization in the presence of water which comprises

(A) an ethylenically unsaturated monomer which is an unsaturated acid,an unsaturated ester, an unsaturated amide, an unsaturated nitrile,unsaturated ether, vinyl pyridine, diethyl vinylphosphonate or sodiumstyrenesulfonate, and

(B) an effective stabilizing amount of a compound of formula I, II, V orVI ##STR1## wherein

R₁ is alkenyl of 2 to 4 carbon atoms, propargyl, zlycidyl, alkyl of 2 to6 carbon atoms interrupted by one or two oxygen atoms, substituted byone to three hydroxyl groups or both interrupted by said oxygen atomsand substituted by said hydroxyl groups, or R₁ is alkyl of 1 to 4 carbonatoms substituted by carboxy or by the alkali metal, ammonium or loweralkylammonium salts thereof; or R₁ is alkyl substituted by --COOE whereE is methyl or ethyl,

R₂ is alkyl of 3 to 5 carbon atoms interrupted by --COO-- or by --CO, orR₂ is --CH₂ (OCH₂ CH₂)_(p) OCH₃ where p is 1 to 4; or

R₂ is --NHR₃ where R₃ is alkyl of 1 to 4 carbon atoms,

n is 2 to 4,

when n is 2, T is --(CH₂ CHR--O)_(q) CH₂ CHR--, where q is 0 or 1, and Ris hydrogen or methyl,

when n is 3, T is glyceryl,

when n is 4, T is neopentanetetrayl,

m is 2 or 3,

when m is 2, G is --(CH₂ CHR--O)_(r) CH₂ CHR--, where r is 0 to 3, and Ris hydrogen or methyl, and

when m is 3, G is glyceryl.

Preferably, R₁ is allyl, methallyl, glycidyl, 2,3-dihydroxypropyl,2-hydroxy-4-oxapentyl or --CH₂ COOH.

Preferably R₂ is methoxymethyl, 2-methoxyethoxymethyl,2-(2-methoxyethoxy)ethoxymethyl, --CH₂ COCH₃, --CH₂ CH₂ COOCH₃ orbutylamino.

Preferably, n is 2, T is is --(CH₂ CHR--O)_(q) CH₂ CHR--, where q is 0,and R is hydrogen.

Preferably, m is 2, G is --(CH₂ CHR--O)_(r) CH₂ CHR--, where r is 0 or1, and R is hydrogen.

The water contents of the instant compositions range from 0.1% to 99% byweight based on the total composition.

The monomers of component (A) have at least one carbon-carbon doublebond capable of undergoing free radical induced polymerization. Suchmonomers are well-known in commerce and comprise a wide variety ofstructural types. Typical examples of such monomers are the unsaturatedacids such as acrylic acid, methacrylic acid and crotonic acid;unsaturated esters such as the acrylates and methacrylates exemplifiedby butyl acrylate, methyl methacrylate, ethyl acrylate, methyl acrylateand vinyl acetate; unsaturated amides such as acrylamide andmethacrylamide; unsaturated nitrites such as acrylonitrile andmethacrylonitrile; unsaturated ethers such as methyl vinyl ether; andmiscellaneous vinyl monomers such as the vinyl pyridines, diethylvinylphosphonate and sodium styrenesulfonate.

Preferably the monomer is acrylic acid, methacrylic acid, butylacrylate, ethyl acrylate, methyl methacrylate, vinyl acetate, acrylamideor acrylonitrile; most preferably acrylic acid, vinyl acetate oracrylonitrile; most especially acrylic acid.

The effective stabilizing amount of component (B) is 1 to 10000 ppm byweight based on the weight of monomer of component (A). Preferably, theamount of component (B) is 1 to 2000 ppm by weight based on the monomerof component (A). Most preferably, the amount of component (B) is 1 to1000 ppm by weight based on the monomer of component (A).

The instant invention also pertains to novel compounds of formula III orformula ##STR2## wherein

E₁ is alkyl of 2 to 6 carbon atoms interrupted by one or two oxygenatoms, substituted by two to three hydroxyl groups or both interruptedby said oxygen atoms and substituted by said hydroxyl groups, or E₁ isalkyl of 1 to 4 carbon atoms substituted by carboxy or by the alkalimetal, ammonium or lower alkylammonium salts thereof; or E₁ is alkylsubstituted by --COOE where E is methyl or ethyl, and

E₂ is alkyl of 3 to 5 carbon atoms interrupted by --COO-- or by --CO--,or E₂ is --CH₂ (OCH₂ CH₂)_(n) OCH₃ where n is 1 to 4; or

E₂ is --NHE₃ where E₃ is alkyl of 1 to 4 carbon atoms,

with the proviso that E₁ is not 2,3-dihydroxypropyl.

Preferably, E₁ is 2-hydroxy-4-oxapentyl or --CH₂ COOH.

Preferably E₂ is methoxymethyl, 2-methoxyethoxymethyl,2-(2-methoxyethoxy)ethoxymethyl, --CH₂ COCH₃, --CH₂ CH₂ COOCH₃ orbutylamino.

The intermediates needed to make the instant compounds are largely itemsof commerce.

The instant invention also pertains to a process for preventing thepremature polymerization of an unsaturated monomer in the presence ofwater by incorporating therein an effective stabilizing amount of acompound of formula I or II described above.

The polymerization inhibitor ether or ester can be introduced into themonomer to be protected by any conventional method. It may be added justupstream of the point of desired application by any suitable means. Inaddition, this mixture may be injected separately into the distillationtrain along with the incoming feed of monomer or through separate entrypoints providing efficient distribution of the activated inhibitormixture. Since the inhibitor is gradually depleted during operation, itis generally necessary to maintain the appropriate amount of theinhibitor ester in the distillation system by adding additionalinhibitor during the course of the distillation process. Such additionmay be carried out either on a continuous basis or by intermittentlycharging fresh inhibitor into the distillation system if theconcentration of the inhibitor is to be maintained above the minimumrequired level.

The nitroxides of this invention are highly water compatible. As many ofthe processes needed to produce and purify the various ethylenicallyunsaturated monomers may have some water present during one of theprocess steps, it is important that the instant stable nitroxideinhibitor be sufficiently water soluble to prevent polymerization in theaqueous phase and yet for the inhibitor to be able to partitionsignificantly into the organic monomer phase for inhibition protectionthroughout the entire process. Undesired premature polymerization mustbe limited or mitigated throughout the purification process to insurethat the reactors, tanks and pipes used to make, store and transport thepurified monomer remain free from high molecular weight polymericmaterial. The instant ether or ester inhibitors are tailored to have thedesirable water compatibility properties needed to bring this about.

The amount of water present will depend on the specific monomer beingstabilized. In the case of monomers of limited compatibility with watersuch as butyl acrylate, the water content will depend on the amountneeded to saturate the ester, only a few percent. On the other hand withwater miscible monomers such as acrylic acid, the amount of waterpossible theoretically much higher.

The following examples are meant to illustrate the instant invention andare not to be construed to limit the scope of the instant invention inany manner whatsoever.

EXAMPLE 1 1-Oxyl-2,2,6,6-Tetramethyl-4-Allyloxypiperidine

A vigorously stirred two phase solution of 30.0 g (0.17 mol)1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine, 29.0 g (0.24 mol) ofallyl bromide, 2.6 g (8 mmol) of tetrabutylammonium bromide, 100 mL of50% aqueous sodium hydroxide and 30 mL of toluene is heated at 70° C.for 90 minutes. The mixture is partitioned between 100 mL of toluene,100 mL of heptane and 200 mL of water. The organic phase is dried overanhydrous magnesium sulfate and concentrated to yield the title compoundas a red oil after column chromatography.

EXAMPLE 2 1-Oxyl-2,2,6,6-Tetramethyl-4-(2-Methoxyethoxy)Piperidine

The title compound is synthesized using the same procedure as describedin Example 1 and using 2-bromoethyl methyl ether in place of allylbromide. The product is isolated as a red oil after columnchromatography.

EXAMPLE 3 1-Oxyl-2,2,6,6-Tetramethyl-4-Glycidyloxypiperidine

The title compound is synthesized using the same general procedure asdescribed in Example 1 and using epichlorohydrin in place of allylbromide. The product is isolated as a low melting red solid after columnchromatography.

EXAMPLE 4 1-Oxyl-2,2,6,6-Tetramethyl-4-(2,3-Dihydroxypropoxy)Piperidine

1.0 g of the compound of Example 3 is heated at 110° C. in 50 mL of 5%aqueous sodium hydroxide for six hours. The mixture is extracted withethyl acetate, and the organic extract is dried and concentrated. Thetitle compound is isolated as a red oil after column chromatography.

EXAMPLE 51-Oxyl-2,2,6,6-Tetramethyl-4-(2-Hydroxy-4-Oxapentoxy)Piperidine

1.0 g of the compound of Example 3 is heated at 60° C. in a solution of0.25 g sodium methoxide in 50 mL of methanol for six hours. The reactionmixture is then partitioned between water and ethyl acetate. The titlecompound is isolated as a red oil after column chromatography.

EXAMPLE 6 1-Oxyl-2,2,6,6-Tetramethyl-4-(Carboethoxymethoxy)Piperidine

0.48 g (20 mmol) of sodium hydride is added to a solution of 3.0 g (17mmol) of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine in 25 mL ofanhydrous tetrahydrofuran. The reaction mixture is stirred under ablanket of nitrogen for one hour. The mixture is chilled to 0° C. and2.9 g (17 mmol) of ethyl bromoacetate is added dropwise. After theaddition is complete, the reaction is stirred for an additional 30minutes during which time a white precipitate forms. The mixture isfiltered and the solvent is removed under reduced pressure. The titlecompound is isolated as an orange solid after column chromatography andmelts at 41-43° C.

EXAMPLE 7 1-Oxyl-2,2,6,6-Tetramethyl-4-(Carboxymethoxy)Piperidine

1.0 g (39 mmol) of the compound of Example 6 is added to a solution of0.2 g sodium hydroxide in 20 mL of 1:1 water/methanol. The mixture isstirred for one hour, carefully acidified with 1% aqueous hydrogenchloride and then extracted with ethyl acetate. The organic extract isdried over anhydrous magnesium sulfate and then contentrated to affordthe title compound as an orange solid.

EXAMPLE 8 1-Oxyl-2,2,6,6-Tetramethylpiperidin-4-yl2-Methoxyethoxyacetate

34.4 grams of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine, 29.6 gramsof methyl 2-methoxyethoxyacetate and 300 mL of heptane are transferredto a 500 mL 3-necked, round-bottomed flask equipped with a mechanicalstirrer, Dean-Stark trap and condenser. Trace amounts of water areremoved by azeotropic distillation. 0.25 mL of tetraisopropylorthotitanate is added to the reaction mixture. The reaction mixture isrefluxed for six hours and the liberated methanol is collected in theDean-Stark trap. The reaction mixture is allowed to cool and is thenpartitioned between 300 mL of ethyl acetate and 300 mL of water. Thephases are separated and the organic phase is washed with water anddried over anhydrous magnesium sulfate. Evaporation of the solventleaves the title compound as a red oil.

EXAMPLE 9 1-Oxyl-2,2,6,6-Tetramethylpiperidin-4-yl2-(2-Methoxyethoxy)Ethoxyacetate

The title compound is synthesized using the same procedure as describedin Example 8 and using methyl 2-(2-methoxyethoxy)ethyoxyacetate in placeof methyl 2-methoxyethoxyacetate. The title compound is isolated as ared oil after column chromatography.

EXAMPLE 10 1-Oxyl-2,2,6,6-Tetramethylpiperidin-4-yl Methoxyacetate

The title compound is synthesized using the same procedure as describedin Example 8 and using methyl methoxyacetate in place of methyl2-methoxyethoxyacetate. The title compound is isolated as an orangesolid by crystallization from heptane and melts at 103° C.

EXAMPLE 11 1-Oxyl-2,2,6,6-Tetramethylpiperidin-4-yl Methyl Succinate

A solution of 6.0 g (35 mmol) of1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine and 11.4 g (78 mmol)dimethyl succinate in 60 mL of heptane is brought to reflux. 0.05 mL oftetraisopropyl orthotitanate is added and the reaction mixture isrefluxed for 16 hours while the evolved methanol is trapped in aDean-Stark trap. The reaction mixture is then concentrated and the titlecompound is isolated as a red oil after column chromatography and meltsat 76° C.

EXAMPLE 12 1-Oxyl-2,2,6,6-Tetramethylpiperidin-4-yl Acetoacetate

The title compound is synthesized using the same procedure as describedin Example 11 but using methyl acetoacetate in place of dimethylsuccinate. The title compound is isolated as a red oil after columnchromatography.

EXAMPLE 13 1-Oxyl-2,2,6,6-Tetramethyl-Piperidin-4-yl Butylcarbamate

0.1 g of di-n-butyltin dilaurate is added to a solution of 1.0 g (5.8mmol) of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine and 0.58 g (5.8mmol) of butyl isocyanate in 10 mL of carbon tetrachloride. Afterstirring for four hours at ambient temperature, the solution isconcentrated and the title compound is isolated as a red oil aftercolumn chromatography.

In the Examples two different test methods are employed to determine theeffectiveness of the nitroxide esters as inhibitors. The method ischosen to simulate different aspects of the purification processes.

Method 1

Acrylic acid is distilled to remove any storage stabilizer present.Stock stabilizer solutions (1.5 mg/mL) are prepared in propionic acid.This stock solution is added to the distilled acrylic acid to give atest solution having 5 ppm of total stabilizer. Aliquots of this testsolution are then placed into three separate reaction tubes. Each tubeis purged with a gas mixture (0.65% oxygen in nitrogen) for ten minutes.The tubes are then sealed and placed in a 110° C. oil bath. The tubesare watched till the appearance of visible polymer formation is observedas a precipitate. Failure times are reported as an average of at leastthree tubes.

Method 2

Test solutions are prepared as in Method 1 except that the stockstabilizer solution is prepared at 0.75 mg/mL giving a test solutionwith 2.5 ppm of total stabilizer. Aliquots (1 mL) of the test solutionare placed into three separate reaction tubes. To each tube is added 0.5mL of toluene and 0.5 mL of distilled water. Each tube is purged asdescribed in Method 1 and then sealed. The tubes are placed in a 90° C.oil bath and heated till visible polymer is observed as a precipitate.Failure times are reported as an average of at least three tubes.

EXAMPLE 18

Following the procedure of Method 1, it is seen that water misciblenitroxides and hydrophobic nitroxides each perform similarly in neatacrylic acid in the absence of water.

                  TABLE 1                                                         ______________________________________                                        Stabilization of Neat Acrylic Acid                                                                 Time to Onset of                                           Compound* of Example Polymerization                                           (5 ppm by weight) (minutes)                                                 ______________________________________                                        none             5                                                              A 220                                                                         Example 8 220                                                               ______________________________________                                         *A is bis(1oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate.             

Each of the nitroxide compounds provide nearly the same stabilizationefficacy to neat acrylic acid.

EXAMPLE 19

Following the procedure of Method 2 where water is present in theacrylic acid, there is a clear difference in the superior stabilizationperformance of the instant water compatible nitroxides of formula I orformula II compared to the hydrophobic nitroxides as seen in Table 2.

                  TABLE 2                                                         ______________________________________                                        Stabilization of Aqueous Acrylic Acid                                                              Time to Onset of                                           Compound* of Example Polymerization                                           (5 ppm by weight) (minutes)                                                 ______________________________________                                        none             30                                                             A 240                                                                         B 130                                                                         Example 1 250                                                                 Example 2 350                                                                 Example 3 490                                                                 Example 4 320                                                                 Example 5 350                                                                 Example 6 400                                                                 Example 7 230                                                                 Example 8 325                                                                 Example 9 520                                                                 Example 10 645                                                                Example 11 600                                                                Example 13 410                                                              ______________________________________                                         *A is bis(1oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate.                  B is 1oxyl-2,2,6,6-tetramethylpiperidin-4-yl octanoate.                  

What is claimed is:
 1. A compound of formula III ##STR3## where E₁ is2-hydroxy-4-oxapentyl or --CH₂ COOH.
 2. A compound of formula IV##STR4## where E₂ is methoxymethyl, 2-methoxyethoxymethyl,2-(2-methoxyethoxy)ethoxymethyl, --CH₂ COCH₃, --CH₂ CH₂ COOCH₃ orbutylamino.
 3. A compound according to claim 2 of which is1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl2-(2-methoxyethoxy)ethoxyacetate,1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl methoxyacetate or1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl methyl succinate.